This invention relates to an rpm control device for an internal combustion engine.
Heretofore, a method of controlling the no-load speed (rpm) of an internal combustion engine to a predetermined value is employed in the art. The purpose of this control is to set the no-load (rpm) speed to a low value thereby to reduce the fuel consumption in the no-load operation as much as possible, and to suppress the variation of the engine speed due to disturbance. Therefore, the control should be high both in response and in accuracy.
Roughly stating, the factors which affect the engine speed (revolution per minute (rpm)) can be classified into a primary group in which the engine speed (rpm) in affected by the variation in no-load loss of the engine itself or by the variation in thermal efficiency of the engine, and a secondary group in which the engine speed is affected by the variation in adjustment gain of suction adjusting means which is used to adjust the engine speed variation caused by the factors of the primary group or it is affected by the variation in density of the air sucked into the engine.
In this connection, Japanese Patent Application (OPI) No. 162340/1984 has disclosed a method in which the suction adjusting means is controlled according to an adjusting signal formed according to the difference between an actual engine speed (rpm) and a target engine speed (rpm), or an adjusting signal outputted according to the difference between an actual suction rate or pressure in the suction pipe and its target value, thereby to cause the engine speed (rpm) to reach the target value. In the method, the adjusting signal (namely, a speed (rpm) adjusting signal) based on the difference between an actual engine speed and a target engine speed is used in the case where the engine speed is affected by the factors of the primary group, and the other adjusting signal (namely, a suction adjusting signal) based on the value which is obtained by integrating the difference between an actual suction rate or pressure in the suction pipe and its target value is used in the case where the engine speed is affected by the factors of the secondary group. Therefore, it goes without saying that the method can adjust the engine speed variation accurately and quickly when compared with a method in which the engine speed only is utilized for the feedback control.
In the above-described conventional method, a suction rate adjusting loop is formed to self-correct an error inherent in its speed control means, and it should be much higher in response than the speed adjusting loop. However, the high response of the suction rate adjusting loop results in the following difficulties: That is, when the engine speed is abnormally decreased because of some disturbance, the suction rate of the engine is quickly decreased, and accordingly the suction rate adjusting signal is quickly increased. When the engine speed is abnormally low, the suction rate depends on the engine speed. Therefore, even if the suction rate adjusting signal is increased, it is impossible to increase the suction rate. As a result, while the suction rate adjusting signal is being increased, the engine is finally stopped.
When, with the engine started again, the speed adjusting loop and the suction rate adjusting loop are activated, the suction rate adjusting signal has been increased to an excessively large value. As a result, the suction rate is excessively large and accordingly the engine speed is abnormally increased. Thereafter, the engine speed is settled at a normal value.